1. Field of the Invention
The present disclosure relates generally to joint replacement surgery, and more particularly to femoral components used in total knee arthroplasty (TKA), in which the femoral components have a modified articular surface.
2. Brief Description of the Related Art
Joint replacement surgery is used to replace worn or damaged articular joint surfaces, thereby allowing the joint to function normally when it would otherwise not be possible to do so. Typically, an artificial joint includes metallic, ceramic and/or polymer components that are fixed to existing bone, which may or may not be resected prior to component affixation. In the case of total knee arthroplasty, a diseased and/or damaged natural knee joint is replaced with a prosthetic knee joint. Knee prostheses typically include a femoral component, a patellar component, a tibial tray or plateau and a tibial bearing insert coupled to the tibial tray. The femoral component generally includes a pair of laterally spaced condylar portions adapted to function similarly to the natural femoral condyles they replace, with articular distal surfaces of the condylar portions interacting with complimentary surfaces formed in a tibial bearing insert.
A goal of total knee arthroplasty procedures is to restore or enhance function of the natural knee while retaining as much of the knee's normal range of motion as possible. A natural knee may have a range of motion from 0° (full extension) to 135° (full flexion), for example. However, a reduced range of motion is sometimes experienced with some known knee prostheses and associated TKA procedures, resulting, for example, in a post operative range of motion of approximately 0-110°. Substantial efforts have been focused on providing “high flex” knee prostheses that offer a range of motion that is as close as possible to the natural knee.
In addition to range of motion considerations, mechanical functioning and longevity are also goals of knee prosthesis design. For example, minimization of joint stiffness and maximization of joint stability throughout the range of motion are desirable in total knee prostheses. Post-operative joint stability is a function of several factors, including surgical technique and implant design. Design efforts have been focused on promotion of prosthesis stability throughout the range of motion, and in particular at the “mid flexion” range of motion, which includes the range of motion around a knee flexion of about 45°.
One known design uses a single radius in the “J-curve” of the knee component condylar portions. The J-curve is the curve of the articular surface of the condylar portions as viewed in a sagittal plane that is medially or laterally offset to intersect with the articular surfaces of each condylar portion. In the “single radius” design, the center of the radius corresponds to the epicondylar axis, which is an axis approximately corresponding with the axis passing through the femoral attachments of the collateral ligaments of the knee. A single radius design typically corresponds to a range of motion from 10° (less than full extension) to 110° (less than full flexion). Thus, a “single radius” design, which seeks enhanced mid flexion stability, does not result in a full range of motion comparable to a natural knee.
Other known femoral components includes J-curves having three or four tangential radii with centers spaced apart from one another, such as shown in FIG. 3. In these designs, the rotational centers of the femur during flexion correspond to the arc centers of the J-curve of the femoral component. Because these radii each have different sizes, the rotational center displaces sharply in the range of flexion corresponding with a transition from one radius to the next, or neighboring, radius. This sharp and large displacement is believed to detract from knee prosthesis stability in these ranges of flexion.
What is needed is a new total knee prosthesis with a high degree of stability throughout a wide range of motion.